FWIW, trying to run an i386 xorg server on amd64 hardware causes errors.

Could you clarify that? I've been running NetBSD/i386 with
i386 xorg server on amd64 hardware for quite some time now,
with a variety of display chipsets including ATI, NVIDIA,
SIS, S3, Cirrus Logic and not seen any issues.
Do you mean running an 32bit xorg server under a 64bit OS/userland?

It was a completely 32-bit system on a 64-bit machine. Twas Ubuntu, but
the fact xorg complained makes it seem relevant.

It might have been something ubuntu/linux specific. Could it
have been detecting the cpu and trying to put somthing into
64bit mode?
Once you have a 32bit NetBSD install running on an amd64 box you
can treat it as any other 32bit box as far as xorg and userland is
concerned...
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